Supercharger Shootout - Brothers In Arms

In This Supercharged Slugfest, We Find Out What 20 PSI Of Boost Is Worth On An '03 Cobra, An '05 GT, And A Shelby GT500.

Richard Holdener

December 1, 2008

Given the current crop of Blue Oval bruisers, it's easy to understand why Ford enthusiasts are so excited. When Ford and Shelby got together and introduced the GT500, the rest of the performance world stood up and took notice. The predecessor of this potent Pony was the aptly named Terminator. Both super stallions offer factory forced induction, but which one is the most potent?

Sure, the stock GT500 takes top honors in terms of displacement and rated power output, but what happens when you modify it? If money was no object, we'd all be driving Ford GT supercars, but since most of us have yet to locate that money tree, the reality is that every penny counts. With that in mind, we decided to include the not-so-supercharged bread-and-butter S197 GT in this shootout.

With its Three-Valve heads, the latest 4.6L motor took a turn away from the less-impressive Two-Valve PI motors of old, while simultaneously offering near-Four-Valve Cobra power numbers. Toss in a dose of variable cam timing and you have the makings of a giant-killer, especially after being treated to the same supercharger applied to the Terminator and GT500.

The question we hoped to answer with this comparison was simple. If supplied the same amount of boost pressure from the same supercharger kit, which combination offered the most power--the 5.4L GT500, the 4.6L Terminator, or the current 4.6L Three-Valve GT motor?

Before you get all up in arms about the big motor having the advantage, know that (like its owners) the Terminator motor backs down to no one. Despite the displacement deficit of nearly one full liter, the Terminator responds to bolt-ons like nothing we've ever seen before. That the all-aluminum mod motor (and overall vehicle) is considerably lighter than the GT500 makes it even more attractive.

While both the Cobra and the GT500 sport four valves per cylinder, don't count out the little Three-Valve motor. What it lacks in displacement or valve count, it makes up for in static compression and variable cam timing. These attributes aren't shared by the Four-Valve contingent, but it must be stressed that the stock Three-Valve rotating assembly wouldn't stand up to the abuse of 20 psi of twin-screw boost without modifications. Given the missing eight valves and normally aspirated origins, we decided to help out the Three-Valve GT motor with a boost-friendly rotating assembly. In went a Cobra crank, forged rods, and stock-compression pistons courtesy of Sean Hyland Motorsport that would allow us to safely crank up the boost on the Kenne Bell blower.

To answer this question, we gathered a stock GT500, an '03 Terminator, and an '05 GT, and subjected them to the rigors of boost. More specifically, all three motors were subjected to exactly 20 psi of boost pressure from the same 2.8L twin-screw supercharger from Kenne Bell. The 2.8L H-series blower was selected for its ability to supply the desired power and boost potential to each combination.

Since we fully expected the combinations to exceed 700 rwhp at 20 psi, we needed a blower that was both capable and comfortable (meaning efficient) at this power/boost level. The 2.8L twin-screw supercharger fit the bill, and Kenne Bell offers both upgrade kits for the Terminator and GT500 motors as well as a complete kit for the Three-Valve.

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Step By Step

KB SC '03 Cobra-20 PSIGiven its humble beginnings of something near 360 rwhp, it's amazing to see this Terminator motor thumping out nearly twice that power level all from a simple blower upgrade. Terminator owners should give thanks for Ford's decision to go the forced-induction route rather than increasing displacement. Running 20 psi of boost, the Kenne Bell supercharged 4.6L Cobra motor thumped out 702 rwhp and 578 lb-ft of torque. In true positive-displacement form, torque production from the supercharged motor exceeded 500 lb-ft from as low as 2,400 rpm all the way to our self-imposed rev limit of 6,700 rpm. The power curve was still headed up at 6,700 rpm, so we know there was more power to be had from this combination.

All three motors were equipped with the new Kenne Bell 2.8L H-series twin-screw supercharger. Capable of supporting well over 1,000 hp, the H-series 2.8L blower was the perfect candidate for this high-boost test.

The secret to the elevated efficiency of the H-series blower is the discharge port. The port timing (how the air exits the blower) helps determine the charge temperature and overall flow. Kenne Bell did a ton of research and development to come up with the new H-series (high-boost) blower designs.

Naturally, all three kits featured air-to-water intercooling, so the idea was to run the same blower, intake manifold (into the blower), and free-fl owing induction system to eliminate any inlet restrictions. All three applications were also treated to the necessary fuel-system upgrades to provide the requisite fuel for the given power level. To level the playing field, naturally each combination was tuned to provide the same air/fuel and timing values. It should be noted that all three were also equipped with Bassani after-cat exhaust systems. With everything in place, all we had to do was crank up the boost and compare the results.

'03-'04 4.6L Terminator702 HP at 6,700 RPM578 LB-FT of Torque at 4,900 RPMPulley Combination 7.5 Crank/3.25 Blower (2.30 Drive Ratio)When first introduced in 1996, the Four-Valve Cobra was welcomed with open arms by Mustang enthusiasts, who saw the DOHC motor as the weapon of choice to do battle with the 5.7L Camaros of the world. As is usually the case, the performance world marched on and quickly left the 300hp Cobra motor in its wake. Ford soldiered on with other normally aspirated Four-Valve motors, which received a power upgrade in 1999, but it wasn't enough to combat the larger and more powerful LS1 motors offered by the General.

What Ford needed was a serious power player; unfortunately, the modular-motor configuration suffered from narrow bore spacing that limited the eventual bore diameter, which in turn, limited the displacement. It was possible to improve the displacement via an increase in stroke length, but even with the additional stroke, 5.0 liters seemed to be the absolute usable limit for a production Four-Valve Cobra motor. Even if Ford decided back then to offer a 5.0 mod motor, would it have been enough to do battle with a 400hp 6.0L LS2?

We now know that Ford never increased the displacement of the Cobra motor (though look for just such a change in 2010); instead it went the forced-induction route. This decision forever changed the lives of many Cobra owners, as the choice of boost over displacement not only increased the factory power output well beyond anything that was possible in normally aspirated trim, but it also offered up a license to produce unlimited street/track horsepower.

Introduced in 2003, the now-legendary Terminator motor offered an exceptional peak power output (underrated at 390 hp), but more importantly, something that was sorely missing in any previous modular motor application, something we like to call torque. Not surprisingly, the normally aspirated Four-Valve motors were somewhat peaky, offering plenty of high-rpm power (especially in modified form), but they lacked the torque production to compete with the larger 5.7L and 6.0L motors over at the Chevy camp. The presence of boost pressure literally transformed the mod motor. The instantaneous boost and attending torque production felt like someone stuffed a 460 underhood. In this case, boost was the perfect replacement for displacement.

The Terminator motors installed in the '03-'04 Cobras were, and continue to be, impressive performers. Compared at 3,000 rpm, a stock supercharged Cobra motor might offer as much as an extra 150 lb-ft of torque over the N/A motor. It's this abundance of torque that pushed the Cobra into a real-world competition with the Corvette. Even more importantly for enthusiasts, Ford built the '03-'04 Cobra motors to withstand plenty of abuse. As good as the Terminator motors are in stock trim, it's the way they respond to modifications that really sets them apart from lesser combinations. Adding 50 hp, 75 hp, or even 100 hp or more to an '03-'04 Cobra motor is as easy as adding an air intake, blower pulley, and performance tune.

Step By Step

Knowing that inlet restrictions kill blower performance, Kenne Bell designed its new Mammoth intake manifolds. Flowing as much as 1,700 cfm, these intakes are what set the Kenne Bell blower systems apart from the competition. It doesn't matter how big or efficient the blower is if the intake manifold restricts the airflow.

Just like the intake manifold feeding the blower, the inlet system must also be designed to minimize airflow restrictions. This 4.5-inch system was employed to ensure plenty of cool air to each of the three motors.

Obviously, the throttle body is a critical portion of the airﬂ ow equation as well. This dual 75mm throttle body offered plenty of airﬂow to the 2.8L supercharger.

With huge power gains just a blower-pulley change away, why on earth would anyone want to replace the factory supercharger on an '03-'04 Cobra? The answer is actually quite simple, as the limiting factor in terms of power production is the supercharger itself. We know from experience that the remainder of the 4.6L Four-Valve motor will withstand a great deal more power, but there is only so much fl ow and power potential hidden inside the factory Roots-style blower.

To extract even more power from the Terminator combination, a blower swap is in order. Obviously, it's also possible to install turbos on the motor, but that's a subject for another story. By now, replacing the factory Roots-style blower on the Cobra has become commonplace, with upgrades from a variety of sources, including Kenne Bell, Whipple, and Eaton TVS. Given Cobra owners' insatiable thirst for power, the blower experts at Kenne Bell configured not only a more efficient twin-screw supercharger design, but one that offered significantly more displacement and therefore more power potential compared to the factory huffer. Where the original blower might be considered a 600hp supercharger, the 2.8L H-configuration twin-screw has already exceeded 1,100 hp on the right application. Think about those figures for a minute. This 2.8L H-series supercharger offers nearly twice the power potential of the stock blower. No wonder these blower upgrades are so popular among Cobra owners.

For our test, the '03 Cobra motor was run through the factory exhaust manifolds and cats feeding a Bassani after-cat exhaust. To keep the testing consistent, all three test vehicles were equipped with a Bassani performance after-cat exhaust. In addition to the Kenne Bell 2.8L H-series blower, the Cobra was upgraded with a dual Boost-a-Pump feeding the stock injectors and a Mafia to eliminate topping out the MAF signal. All three test motors were also equipped with a free-flowing inlet system in front of the supercharger, including a dual 75mm throttle body, a 4.5-inch inlet tube and mass air meter, along with the oval fl expipe from the Ford GT.

Naturally, the 2.8L blower was fed using a new Mammoth intake manifold. In short, every effort was made to eliminate inlet restrictions that would limit the power potential of each combination. Combining a 7.5-inch crank pulley with a 3.25-inch blower pulley (a 2.30 drive ratio) resulted in the desired 20 psi of boost. Tuned to perfection on race fuel, the combination produced 702 hp and 578 lb-ft of torque. Can you imagine what kind of fun you could have with a Cobra making that much power?

'05-Up 4.6L GT704 HP at 6,500 RPM607 LB-FT of Torque at 4,700 RPMPulley Combination 6.5 Crank/3.00Blower (2.17 DRIVE RATIO)What car combines the good looks of a '69 Mach 1, the performance of a 428 Cobra Jet, and the driveability and comfort of a modern car? The answer is the '05-up Mustang GT. The retro-modern styling obviously ranks high on the cool scale, and the amount of mods one can make is matched only by the Fox-body Mustang.

Were the new Mustang a less-than-stellar success, Chevy and Dodge wouldn't have been so quick to jump on the nostalgia bandwagon with the yet-to-be-released Camaro and Challenger models. Now more than three years old, the GT still looks great, especially decked out in stripes and spoilers. Sporting near-Cobra power (naturally aspirated, of course), is a 4.6L Three-Valve motor with variable cam timing. This new modular motor nearly matches the output of the older Four-Valve Cobra motors using one less valve per cylinder. The only problem with the new GT is that the impressive Three-Valve motor has to push a ton of extra weight. The Mustang GT has bulked up considerably since the days of the 5.0L, and all that weight takes a toll on performance.

Step By Step

KB SC '05 GT at 20 PSIThe real surprise of the group was how well the '05 Three-Valve GT did against its Four-Valve brethren. Despite the handicap of having one less valve per cylinder, the Mustang GT motor responded well to the boost. Apparently, the higher static compression and variable cam timing more than made up for the valve deficiency compared to the Terminator motor. The peak power exceeded 700 at the same 20 psi, but the torque production on the Three-Valve motor topped the 600 lb-ft mark, some 28 lb-ft better than the Cobra motor. Even more impressive was the fact that the Three-Valve motor required less blower speed to achieve this power output. From the looks of this test, the '05-up GT motors can hold their head high at the Ford family reunion.

Changing the blower speed and boost level on each combination required changing the blower (and crank) pulleys. Each combination was adjusted to achieve a maximum of 20 psi.

While the Terminator and GT500 motors made do with their stock injectors (and plenty of fuel pressure), the GT motor was upgraded with a set of 60-lb/hr injectors.

Given its humble, normally aspirated beginnings, the Three-Valve GT motor was never designed with forced induction in mind. Where the Terminator and GT500 motors would easily withstand 20 psi of boost, we could hardly expect the same from the GT motor. To level the playing field, we augmented the GT motor with a suitable reciprocating assembly. The stock components were removed and replaced with forged components from Sean Hyland Motorsport. This included a Cobra steel crank, forged connecting rods, and forged pistons that duplicated the stock compression ratio. We could have dropped the static compression, but we wanted to run the test in stock configuration with the only addition being the required stronger rotating assembly. Besides, with only three valves per cylinder, the GT motor was already at a deficit compared to the Four-Valve Cobra and bigger GT500 motor. We hoped the higher static compression and variable cam timing would help offset the valve count. As it turned out, the Three-Valve motor more than held its own.

All three of the supercharger kits run on the GT, Cobra, and GT500 featured air-to-water intercooler systems. The Kenne Bell supercharger was installed along with the Mammoth intake manifold, 4.5-inch intake tubing and MAF and the dual 75mm throttle body. The GT also required a Mafi a, FRPP fuel pump kit (with dual BAP) and 60-pound injectors. The GT motor was equipped with a 6.5-inch crank pulley and 3.0-inch blower pulley to produce the requisite 20 psi of boost. Note that the blower speed (drive ratio of 2.17) was lowest on the GT motor.

Fed 20 psi of boost from the Kenne Bell blower, the Sean Hyland GT motor produced 704 hp at 6,500 rpm and 607 lb-ft at 4,700 rpm. Like the Terminator and GT500 motors, the power output was still climbing at 6,500 rpm. Though the peak power outputs were similar between the two 4.6L motors, it was the Three-Valve that produced much more average power. Torque production on the GT motor was up by 28 lb-ft over the Terminator. Is this enough to make up for the weight difference between the '03 Cobra and the new GT? Only a trip to the strip would settle that argument, but for now, know that the GT motor can more than hold its own against the Four-Valve contingent--at least those displacing 4.6 liters, that is.

'07 5.4L GT500756 HP at 6,200 RPM684 LB-FT of Torque at 4,800 RPMPulley Combination 7.5 Crank/2.75 Blower (2.73 Drive Ratio)Looking back to the recent past, we see the T-bird Super Coupe as a possible predecessor of the current crop of supercharged Stangs, but credit probably belongs to the Lightning for starting the current mod-motor mania. Though saddled with the more-restrictive Two-Valve cylinder heads, the Lightning was blessed with an abundance of displacement compared to the Mustangs.

With 5.4 liters of displacement (330 ci), the Lightning motor combined displacement with the immediate boost response of the Roots-style blower to produce one exceptionally torquey machine. We know now that Ford soon introduced the Terminator and all but one-upped the valve-challenged Lightning motors, but the Terminator was still down on displacement. It wasn't until Ford introduced the ultra-exotic, all-aluminum GT supercar that we saw the proper combination of displacement with the high-fl owing Four-Valve heads.

Luckily for enthusiasts, Ford saw fit to drop a version of that motor in the Mustang we now know as the GT500. Though missing the aluminum block, the dry-sump oiling, and the twin-screw supercharger of the GT motor, the GT500 still offered plenty of performance. Compared to the previous Terminator motor, the GT500 mill offers more displacement, a larger Roots-style blower, and free-flowing cylinder heads. Time and technology march on, and we enthusiasts get to reap the rewards.

Step By Step

KB SC GT500-20 PSIWhat could be better than combining a Four-Valve motor with efficient boost from a twin-screw supercharger? That's easy--combining an even bigger Four-Valve motor with the blower. If you review the graphs of the previous 4.6L motors, you'll see that torque production exceeded 500 lb-ft as low as 2,300 rpm (2,400 rpm for the Terminator). By contrast, torque production from this larger 5.4L motor exceeded 600 lb-ft at the same engine speed, continuing to do so to nearly 6,500 rpm. With a peak torque reading of 684 lb-ft, the 5.4L clearly demonstrated its superiority. Like the previous mod motors, the GT500 clearly was not finished making power at 6,200 rpm, so there's more power to be had from this combination by extending the engine speed. Just remember that the piston speed of the 5.4L is dramatically higher than the smaller 4.6L, so care must be exercised when combining high boost and high rpm.

Much like the Terminator, the limiting factor on the GT500 motor is the supercharger itself. Despite being larger than the blower employed on the 4.6L Cobra, the Eaton supercharger was designed for the GT500 with a specific power and price goal in mind and therefore has certain limitations. It wouldn't make any sense for Ford to install a blower capable of 1,000 hp on a motor that only needs to make 500 hp. Limiting the blower capacity actually works in Ford's favor, as this helps minimize the warranty claims caused by enthusiasts cranking up the boost. Just imagine how many motors would be in for warranty if you could go from 500 hp to 1,000 hp with a simple pulley change? The factory blower was chosen for its combination of power potential and price point, as the Roots-style configuration is considerably less expensive than a comparable twin-screw supercharger. This is why Ford employed the twin-screw design on the more expensive Ford GT. A twin-screw design is more efficient than a Roots-style blower (including the new twisted-lobe TVS blowers), but it is more expensive.

While Ford chose the Roots-style blower for the GT500 motor, the Kenne Bell upgrade featured the more efficient and larger 2.8L H-series blower. What could be better than a bigger, more efficient motor being fed by a bigger, more efficient supercharger? Running the KB blower up to 20 psi on the larger GT500 motor produced exceptional results.

Achieving 20 psi on the 5.4L required a drive ratio of 2.73 (7.5 crank/2.75 blower). Basically it takes more blower speed to fill the larger motor. Run at 20 psi, the GT500 motor produced 756 hp at 6,200 rpm and 684 lb-ft at 4,800 rpm. The combination of increased displacement and blower speed resulted in a serious jump in torque production, too. Compared at 2,300 rpm, the GT500 bettered the smaller 4.6L motors by a solid 100 lb-ft of torque. Can you harness 600 lb-ft at just 2,300 rpm? Probably not, but it's always better to have more and need less than the other way around. Besides, the GT500 needs the extra power to offset the extra curb weight.

With over 700 hp, any of these brothers in arms would make for one serious street machine, but run at the same boost level, it looks like there really is no replacement for displacement.

'03 VS. '05 VS GT500--Same Drive RatioFor our main story, we supplied 20 psi of boost to the three different test Mustangs. Achieving the 20 psi of boost required different drive ratios (pulley combinations) for the three different motors. The reason for this is a combination of displacement and efficiency. The larger 5.4L required more blower speed to produce the desired 20 psi. The same can be said for the low-compression Terminator motor, as the Four-Valve Cobra motor required more blower speed than the Three-Valve combination to produce the same 20 psi.

What if we were to run the 2.8L blower on all three combinations at the same speed (same pulley combination) rather than try to select a particular boost level? For this test, all three combinations were equipped with an identical drive ratio of 2.30. With the blower spinning the same speed, it was the Three-Valve motor that came out on top, bettering even the mighty GT500 motor. Of course, running the blower at the same speed on all three combinations resulted in dramatically different boost levels. The power output and boost level (20 psi) of the Terminator remained the same (it already had a 2.30 drive ratio), while the boost level of the GT500 dropped to 16 psi. The change in drive ratio to the Three-Valve motor increased the boost pressure to 24 psi.